Method and apparatus for pumping volatile liquids



Nov. 20,*1951 K. D. MGMAHAN Erm. 2,575,923

METHOD AND APPARATUS FOR PUMPING VOLATIL LIQUIDS Filed Deo. 29, 1948 Patented Nov. 20, 1951 ,METHOD AND APPARATUS Foa PUMPING l voLA'rILE LIQUms y i Kenton D. McMahan, Schenectady, and Kenneth A. Darrow, Sprakers, N. Y., assignors to General Electric Company, a. corporation of New York "Application December 29, 1948, Serial No. 67,946

7 This invention relates 'to an improved method and 'apparatus for pumping volatile liquids; especially useful in -connection with the -fuel supply systems of gas turbine combustors, though of course not necessarily limited to such. appli-i cations.

vParticularly-inY connection withv small gas turbine powerplants having anet output less than 500- P., where the rotational speeds are extremely high and simp'licity and low first cost are important, it is highly desirable to employ a 'fuel pump having a single rotating element capable of operating at the rotor speed of the powerplant, so that no speed change gearing is required. Heretofore, centrifugal type pumps have been considered impracticable for such service because of their unstable and unreliable operation when pumping comparatively light hydrocarbons such as kerosene, which tend to ,volatilize, entrain, and release' dissolved air, with resulting 'excessive foam formation when put through a conventional centrifugal' impeller. Such-pumps become inoperative at high speeds and have heretofore been limited to low pressure applications. An objectof the presentinvention is: therefore to provide animproved liquid `supply system employing a centrifugal pump, especially arranged to4 separate dissolved gases from the liquid, and then separate-the resulting foam so as to' deliver gas-,freejliquid .to ,the fuelv spray nozzle of the combustor. l

es further object is to provide an improved system for supplying fuel to a-wide range dual yorV duplex type spray nozzle requiring-separate supplies of,liquidat-differingxpressure levels.

f Qther objects'andadvantages will be. apparent from the following descriptionl ,taker-rin connectionwiththe accompanying drawing in which the single gitrerepresent's schematically .a fuel system incorporating vmy improved centrifugal pump., '..=..-`\'\.."4 ,Thel centrifugal pump, indicated generally at -I has@ esinewhih mayanvenintly ae formed in two sections 2 and 3 secured together by-suit= able threaded fastenings 4.v Thev casing portion B ,denes an inlet passagei to which 'liquid `is Supplied by a suction.. conduit 6: communicating with the lower portion Vof Lthe liquid fuel reservoirV l and having -a shut-off .valveiGaf Tank 'l is provided with an atmospheric. vent :1a. Pump casing. portion 3 is provided with 4'a discharge conduit ..8 adapted to supply liquidato the Ifuel. spray nozzle. indicatedgenerally at 9 and ythrough a branch conduit 4Ila containing a check valve I0 tela starting fuel tank oraccumulator II; in a 7 Claims. (Cl. 103-2) 2 manner described more particularly hereinafter. A bypass conduit 8b in parallel with the check l valve I contains a manually controlled start--` ing valve I2. .l

The pump casing portion 2 is providedwith 'a high pressure outlet supplying liquid to a con-e duit I3 for actuating jet pumps Il, I5. and ,a second port arranged to deliver liquid at a somewhat lower pressure to -a conduit IE, which lis also connected with the fuel spray nozzle 9.

While the spray nozzle 8 may be of the so.

' called duplex. type, as described in the co-e pending applicationof .C4 D. Fulton and D. C. Ipsen, Serial No. 622,604, filed October 16, 1945.' and assigned to the same assignee as the present application, it is preferably of the dual vortex type disclosed in the copending application of.` Kenton D. McMahan, Serial No. 72,216, filed January 22, 1949 now Paten-t 2,551,276 dated May l, 1951. Wide range nozzles of these types have vortex or spin chambers to which liquid is sup'- plied at low rates of flow through a first setof orifices referred to as small slots, designed to give an effectively atomized spray at fuel pressures corresponding to the ignition and idling, speeds of the powerplant. For increasing speeds and loads, additional fuel is supplied through a separate conduit to a second set of large slots." 'I'hewmechanical details of such nozzles are not material to an understanding of the present irlvention; and it need only be noted that the conduit 8 supplies liquid to the 'small slots in nozzle 9, whileoonduit I6 supplies the large slots. While the details of the combustor structure are likewise unnecessary to an understanding of the present invention, it may be noted that the combustor shown is disclosed fully in the coperid.` ing application of Kenton D. McMahan, Serial No. 705,866, led October 26, 1946, now Patent 2,510,645 dated June 6, 1950 and assigned to the same assignee as 4the .present application.y The detailed construction of the improved centrifugal pump I is as follows.v

The, pump .rotor includes a shaft I'I, which may be an 4integral .extension of the main rotor a journal bearing 20, and in casing 2 by a second journal bearing 2l, the latter bearing consisting of a composite shell having an outer cylindrical portion 22 pressed or otherwise secured in the casing 2, with an inner cylindrical sleeve 23' de.-

lining with the outer portion 22 longitudinal and Y radial passages' 24, the purpose of which will be seen hereinafter. Shell 23 may be spaced from the outer shell'22 by means of a plurality of circumferentially spaced ribs 23, which lof c ourse may be formed integral with either the outer member 22 or theinner member 23. lThe inner sleevey 23 is provided with a babbitted or silver alloy bearing surface indicated at 23.

Pressed, keyed, or otherwise secured to shaft I1 is a centrifugal rotor indicated generally at 21. This is actually a composite rotor having at the left-hand side thereof circumferentially -spaced radially extending blades 23 forming a first centrifugal pumping element, receiving liquid from the suction passage 3. Actually, the

impeller 23 is not intended primarily to serve as creased pressure adjacent the inlet of the centrifugal element 23, as compared with the supply pressure in the suction conduit 6. Impeller 23 serves to violently agitate the incoming liquid at a reduced pressure; and both the reduced pressure and the mechanical agitation tend to separatedissolved gases from the liquid. Thus the impeller 23 delivers a mixture of liquid and foam into the annulary dischargepassage 29 formed around the impeller in the casing portion 3. From passage 23, this mixture of foam and liquid is delivered by a radially extending conduit 30 and a cored passage 3l formed in casing portion 2 to the inlet chamber 32 `of a second centrifugal pumping element formed at the other end of the `rotor 21 and including circumferentially spaced radially extending blades 33 carrying a rather complex shroud structure consisting of a rst portion 34 defining a first circumferential groove containing sealing rings 35|and a second circumferential chamber 33, the latter communicating withtheilow paths dened by blades 33'by means of a few small radial drilled holes 31. This shroud portion 34 is connected to or formed integral with other shroud portions including a radially extending wall 33 with an enlarged central'annular portion 33 having an outer circumferential groove containing packing rings 43. It

tion of the incoming liquid, the number of blades 33 is kept to theabsolute minimum required in order to effectively maintain a forced vortex. Thus, although both impellers serve to increase the pressure of the liquid, the element 23 serves an extremely important function of agitating the liquid at a comparatively low pressure to eiIect separation o f dissolved gases, while the second element 33 acts as a centrifuge. It will also be observed that the first impeller is comparatively small while the second is much larger than actually needed to pump the flow-of liquid required by the nozzle 3. 'Ihus the mixture of liquid andv Vfoam remains in the large centrifuge chamber formed by the inner edges of blades 33 for a sufilcient length of time so that foam and liquid have an opportunity to separate. The blades 33 are comparatively short in a yradial direction thus providing a large central centrifuge chamber adjacent the shaft.

The impeller 33 is arranged to supply liquid to a plurality `of separate annular discharge chambers 44, 43, 43 deilned by a ring member 41, which may be pressed or otherwise secured in ak cylindrical recess in the housing portion 2. As will be apparent from the drawing, discharge chambers 44. 45, are supplied with liquid at the full discharge pressure of the impeller 33. by way of a plurality of circumferentially spaced openings 43, 43, while the collector passage 43 receives liquid from chamber 3s by way of circumferentially spaced; openings 33. It will also be apparent that the shroud portion 34 of the impeller 33 has circumferential edge portions defining close clearances with the stationary member 41. this clearance being sealed by packing rings 3l. It

should also .be noted thatythe chamber 33 receives liquid by way of drilled holes 31 from the impeller 33 at a radius substantially less than the full tip radius of the impeller. 'I'hus the liquid supplied to chambers 33, 46 is at a substantially lower pressure than that supplied to; chambers 44, 43. It will also be seen in the drawing that collector 44 communicates with a cored mssage 3i in housing 3, which in turn communicates with conduit 3, while collector passage Il will be understood by those skilled in the art that Y thefpa'cking devices 35, 43 are of a type consistingo! a plurality of separate ring members havintan outer circumferential surface carefully machined to sealingly engage a cooperatinganh ular surface in the housing, while the fluid presdiiference across the sealing rings forces them axially in one direction so as to sealingly 'engage a radial surface of the rotor member. This type of sealing device has become well known in the art and need not be further described here.

In'order to limit the flow of impeller 23 so as to keep it` within its stable range of operation, the passages 33,` 3l contain a flow restricting orifice in the form of an opening 4I formed in a plate or diaphragm 42, which may be conveniently clamped by threaded fastenings 4 between the casing halves 2, 3. This plate 42 is annular, and is provided with a second restricted opening 43 serving a purpose noted hereinafter,

It is to be particularly noted that, while the impeller 23 has a substantial number of blades,

communicates with conduit I3, and passage 43 with conduit i3. The aggregate area of the discharge openings 43, 43, 31 is made small to restrict the rate of flow of liquid from impeller 33, thus insuring that the liquid will remain in the centrifuge chamber long enough to permit separation of the gas and liquid. From the above description of thestructure.l will be seen that my improved pump is specihcally designed to perform the 'following process: n (l) Liquid from supply tank 1, being substantially at ambient pressure by reason of vent 1a, is supplied to the first impeller 23, which `subjects the incoming mixture of liquid and entrained gases to a sub-atmospheric pressure and at the same time thoroughly agitates they mixture so as to beat the dissolved gases out of sothere will be a pressure drop across the orifice 4I, the static pressure in the passage portion 3l will be even lower than that in the passage portion 30 upstream from orifice 4I. The strong suction effect produced by the impeller 33 in the conduit 3l, in combination with the flow restricting effect of orifice 4I, also insures that the pressure will below in the passage 3 l, which comprises the major part of the passage between the impellers. Also the passages 30, 3| are of relatively large cross-section area so that the liquid velocities will be low so as to give a time interval in which the gases can further separate from the liquid.

(3) Through the centrifuging action created in the forced vortex whirl maintained by impeller blades 33, the gas-free liquid is thrown outwardly, while foam and entrained gases tend to remain at the radially inner portion of the vortex. While, as noted above. the static pressure in the inlet chamber 32 of the second impeller is comparatively low. it is still substantially above atmospheric pressure, with the result that foam and entrained gases are bled off through the axial passages 25 in the bearing `sleeve member 24, thence through a return conduit 62 to the upper portion of reservoir 1. It will be understood that the reservoir 1 is made of suilicient capacity that the liquid-foam mixture will have an opportunity to cool and separate to some extent, so that comparatively foam-free liquid is drawn off ad- 4jacent the bottom of the tank through suction conduit 3.

Entirely gas-free liquid at a pressure on the order of 600 pounds per square inch is supplied from collector passage Il through orifice 43 and chamber' 5I to conduit l, thence to the small slots of nozzle 3. Likewise. liquid at the full discharge pressure of impeller 33 is supplied from collector passage 45 to conduit I3 for actuating the jet pumps I4, I5, in the manner described hereinafter. Liquid at full discharge pressure also passes from conduit 8, through branch conduit la, past check valve I to the starting fuel tank or accumulator II. It is to be noted that the orifice I3 in plate member 42 is arranged to be covered by a feather" or "fiapper member 42a, forming another check valve for preventing flow of-high pressure liquid from the accumulator tank II backward through chamber 5I into the pump 3l. Y

,The `iet pump I5 actuated by high-pressure liquid received through conduit I3, is a scavenger for drawing ol! through conduits i3, 54 any liquid leaking past the shaft seals I8, I9. The mechanical construction of Jet pump Il will be obvious from the drawing.

The jet pump I l serves a very different purpose. As indicated above, it is actuated by high pressure motive liquid received from conduit I3, and has a suction conduit vIsa communicating with the large slot supply conduit Il. The function of the pump Il is to variably draw ofi liquid from conduit Ii so as to alter thepressure therein in a desired manner, as determined by a manual control valve Ila which regulates the discharge of the jet pump Il into the common return conduit Il.

The integrated operation of the entire system may be summarized briefly as follows. Ordinarily when the power-plant is shut down, the manual control valve Ila will be open. the starting valve i2 will be closed, and the accumulator tank I I will be fully charged. To start the powerplant, the rotor. including the pump rotor Il, is brought plant (not shown) to the combustor at a rate sufficient to initiate combustion. The igniting device in the combustor is then energized and the starting fuel valve I2 opened, so that the stored pressure in accumulator tank II causes liquid to flow through the bypass conduit 8b and conduit 8 to the small slots of nozzle 9. Any flow of this starting fuel backward through the pump 33 is prevented by the check valve 42a. This supply of starting fuel to the small slots of nozzle 9 produces a fuel spray, ignition is effected, and the powerplant now operates under its own power. As the rotor comes up to idling speed, the discharge pressure of pump 33 becomessufficient to force liquid past check valve 42a so that the small slots are supplied through conduit l, and the tank II is re-charged through conduit la past check-valve I0. The manual starting valve I2 may now be closed, and the plant will be selfsupporting at idling speed with the small slots supplied from the pump 21. Since the manual control valve Ila is wide open, the jet pump Il willV be effective to draw off the fuel from conduit I6 so that there is no flow to the large slots through conduit I6. To accelerate the powerplantto full rated speed the manual control valve Ila is slowly closed, with the result that less liquid is bled off through conduit lia and the pressure builds up in conduit Ii so that an effective flow is produced to the large slots. This additional fuel flow to the large slots results in increased combustor temperatures, increased energy available to the turbine, and therefore a higher speed. Thus by suitably controlling the manual valve Ila, the speed and output of the plant can be varied. When the valve Ila is opened wide, the fuel flow will return to that value required for idling or "no-load operation,

with this minimum supply of fuel being furnished by the small slots above. Thus the dual or duplex nozzle i is effectively controlled by a single, simple control valve. To stop the powerplant it is only necessary to positively shut off the supply of fuel by means of the valve 6a.

A fuel system in accordance with my invention has been found to be very effective in supply-y ing and controlling a small gas turbine powerplant of about 300 H. P., having a rated normal speed of around 40,000 R. P. M., an idling speed in the neighborhood of 10,0000 R. P. M., and a fuel pump with a maximum discharge pressure in the neighborhood of 600 lbs. per square inch. Em'- cient use of a centrifugal type fuel pump in such an application is made possible by our novel method and apparatus for separating entrained gases from the liquid fuel and insuring that completely gas-free liquid is supplied to the fuel nozzle.

While one modification has been specifically illustrated herein, it will be apparent that :many changes may be made in the apparatus employed. For instance, instead of the simple starting fuel tank II, a liquid accumulator of a well-known type having a flexible diaphragm or bladder., with a compressible gas on one side thereof. may be used. Also, the method of operation described herein may be performed by very different apparatus. For example, the agitator 28 and centri- Iuge 33 need not be arranged on a common rotor but might be entirely separate machines, separately driven by any suitable means. We of course intend to cover by the appended claims all such modifications as fall within the true spirit and scope of the invention. l

What we claim as new and desire to secure by Letters Patent of the United States is:

l. In a continuous method of delivering a liquid at a high pressure free of dissolved gases and foam, the steps which include drawing liquid continuously from a reservoir, agitating the liquid violently at a pressure'below that in the reservoir to effect separation of gases from the liquid, causing the resulting mixture of form and liquid to.ow at comparatively low velocity and at a relatively low pressure through a conduit of such length as to provide an appreciable time interval to permit the gases to further separate from the liquid, the low velocity and pressure in said conduit reducing the tendency of the gases to go back into solution in the liquid, subjecting the mixture of liquid and foam to strong centrifugal forces with aminimum of agitation whereby gas-free liquid is centrifuged from the foam, and separately drawing off the gases and foam adjacent the center of the centrifuge and drawing ofi gas-free liquid at the outer periphery of the centrifuge at a limited rate, so that the mixture is retained in the centrifuge chamber for an appreciable time interval to effect thorough separation of liquid and gas.

2. Apparatus for supplying substantially gasfree liquid at a high pressure including a liquid reservoir, a first high-speed rotor having fluid flow passages for violently agitating liquid from said reservoir at a pressure below that in the reservoir and then raising it toa pressure above that in the reservoir, a second high-speed centrifugal rotor defining a centrifuge chamber in which the mixture of foam and liquid is caused to whirl as a forced vortex with a minimum of agitation, stationary walls defining a conduit of substantial length connecting the discharge of the first rotor with the inlet of the second rotor and including a flow-restricting orice adjacent the exit of the first rotor to limit the discharge therefrom, wherebyl the major ,portion of said conduit is maintained at a pressure below the discharge pressure of the first impeller and the tendency of the gases to re-enter solution in the liquid is reduced, walls defining a conduitconnecting the centrifuge chamber with the reservoir for drawing ofi foam and entrained gases from the radially inner portion of said centrifuge chamber yand returning the gas and foam mixture to the reservoir, and other restricted con-l duit means communicating with radially outer portions of said centrifuge chamberfor drawing of substantially gas-free liquid at a high prefisure.

3. In a system for delivering a supply of liquid at twodiferent pressure levels, the combination of a liquid supply reservoir, a two-element centrifugal pump including a first rotor member supported on a shaft with at least one shaft seal device, said rotor having circumferentially spaced radially extending blades defining flow paths receiving liquid from said reservoir and agitating it violently at a pressure below that in .the reservoir to effect separation of entrained gas from the liquid, said first rotor having a discharge l pressure above that in the reservoir, said centrifugal pump also including a second rotor element defining a centrifuge chamber of substantially larger volume than the aggregate volume of the flow paths ofthe. first rotor. whereby the 8 mixture of liquid and foam is retained in the centrifuge chamber for a time interval sumcient to effect centrifugal separation of foam andgas- 1ree liquid, walls defining a first conduit oonnecting the discharge of the first rotor with the inlet of the second rotor and including a flow restricting orifice adjacent the exit of the first rotor for limiting the discharge .thereof s o that said rotor is kept within its stable operating range and the major portion of said conduit .Lis maintained at a pressure substantially belw the discharge pressure of the first rotor but above the reservoir pressure, walls defining a passage com municating with the radially inner portion lof said centrifuge chamber and including conduit means connected to the reservoir whereby foam and entrained lgases are forced to return=to the reservoir by the pressure at the second impeller pump, a second jet pump actuated by liquid at' the full pressure of the second impeller and having a suction conduit' connected to withdraw leak age liquid from said shaft seal device, 'return conduit means connecting the discharge ports of both jet pumps with the reservoir, and means for supplying liquid during the starting` cycle ofthe system including a liquid accumulator, fourth conduit means connecting said second conduit with the accumulator, said second con# duit `including a first check-valve adjacent the centrifuge chamber outlet for preventing backward flow thereinto, said fourth conduit having a second check-valve preventing discharge of the accumulator, and a controllable valved 'bypass around said second check-Valve" for effecting di's' charge of liquid from theaccumulator'irito th second conduit. Y y' 4. In a system for delivering a supply of liquid at two vdifferent pressure levels, the combination of a liquid supply reservoir, a first pump rotor having circumferentially spaced fluid 'passages to 4receive liquid from said reservoir 'andagitate the liquid violently at a pressure below thatfof y the reservoir to effect separation of entrained gas from the liquid, said first rotor having a discharge pressure above that inthe reservoir, a second centrifugal pump element having a minimum number of short radially extending blad4 16113' ing a centrifuge chamber of a substantially larger volume than the aggregate volume lof the flow paths of the first rotor, whereby the mixture. of liquid and foam is retained in, the centrifuge chamber for a time interval sufflcienttofeffect centrifugal separation of foam andl gas-free liquid, walls defining a first conduit connecting the discharge of the first rotor with'the inlet of the second rotor and including flow restricting means adjacent the exit of the first rotor for limiting the discharge thereof and maintaining the major portion of said conduit at a pressure below the/discharge pressure of the first rotor but above reservoir pressure,V walls defining a conduit connected to the reservoir and 7c.omrnulnicating with the central portion of the centrifuge chamber whereby the foam and entrained gases are forced to return to the reservoir by the pressure differential between the second impeller inlet and that in the reservoir, second restricted conduit means communicating with said centrifuge chamber at its maximum radius for supplying gas-free liquid at the full discharge pressure of the second impeller, third restricted conduit means withdrawing gas-free liquid from the centrifuge chamber at an intermediate radius for supplying liquid at a lower pressure, valved conduit means for withdrawing liquid from the third conduit at a variable rate, and means for supplying liquid during the starting cycle of the system including a liquid accumulator, fourth conduit means connecting the second conduit with the accumulator, a first check valve adjacent the centrifuge chamber outlet for preventing backward flow thereinto from the second conduit, said fourth conduit having a second check valve preventing discharge of the accumulator, and a controllable valved bypass around the second valve for effecting discharge of liquid from the accumulator into the second conduit for supplying fuel when the discharge pressure of the second impeller is insuiiicient to open said first check valve.

5. In a system for delivering a supply of liquid at two different pressure levels, the combination of a liquid supply reservoir, a first pump rotor having circumferentiallyrspaced fluid passages to receive liquid from said reservoir and agitate the liquid violently at a pressure below that of the reservoir to effect separation of entrained gas from the liquid, said first rotor having a dischange pressure above that in the reservoir, a second centrifugal pump element having a minlmum number of short radially extending blades defining a centrifuge chamber of substantially larger volume than the aggregate volume of the' flow paths of the first rotor, whereby the mixture of liquid and foam is retained in the centrifuge chamber for a time interval sufficient to effect centrifugal separation of foam and gas-free liquid, walls defining a first conduit connecting the discharge of the first rotor with the inlet of the second rotor and including flow restricting orifice means adjacent the exit of the first rotor for limiting the discharge thereof and maintaining the major portion of said conduit at a pressure below the discharge pressure of the first rotor but above reservoir pressure, walls defining a conduit connected to the reservoir and communicating with the central portion of the centrifuge chamber whereby the foam and entrained gases are caused to return to the reservoir by the pressure differential between the second impeller inlet and that in the reservoir, second restricted conduit means communicating with said centrifuge chamber at its `maximum radius for supplyins gas-free liquid at the full discharge pressure of the second impeller, third restricted conduit means withdrawing gas-free liquid from the centrifuge chamber at an intermediate radius for supplying liquid at a lower pressure, and valved conduit means for withdrawing liquid from said third conduit whereby the total now of uid delivered may be regulated.

6. a continuous method cf delivering a liquid under pressure free of dissolved gases and foam,

l 10 the steps which include drawing liquid continuously from a reservoir, agitating the liquid violently at a reduced pressure to effect separation of gases from the liquid, causing the resulting mixture of foam and liquid to flow at comparatively low velocity and at a relatively low pressure through a conduit of such length as to provide an appreciable time interval to permit the gases to further separate from the liquid, the low velocity and pressure in said conduit reducing the tendency of the gases to go back into solution in the liquid, subjecting the mixture of liquid and foam to strong centrifugal forces whereby gas-free liquid is centrifuged from the foam, and` separately drawingoif the gases and foam adjacent the center of the centrifuge and drawing off gas-free liquid adjacent the outer periphery of the centrifuge at a limited rate, so that the mixture is retained in the centrifuge chamber for an appreciable time interval to effect thorough separation of liquid and gas. i

7. In a system forI delivering a volatile liquid under pressure, the combination of a liquid supply reservoir, a first pump rotor adapted toreceive liquid from the reservoir and agtate the liquid violently at a reduced pressure to effect separation of entrained gas from the liquid, a second centrifugal pump rotor forming a centrifuge chamber of substantially larger volume than the aggregate volume of the flow paths of the first rotor whereby the mixture of liquid and foam is retained in the centrifuge chamber for a time interval sufcient to effect centrifugal separation of foam and gas-free liquid, walls f defining a first conduit of substantial length connecting the discharge of the first rotor with the inlet of the second rotor and including orifice means adjacent the exit of the rst rotor for re strictlng the discharge therefrom and maintaining the major portion of said conduit at a pressure below the discharge pressure of the first rotor, walls defining a second conduit communieating with the central portion of the centrifuge chamber and connected to discharge into a lower pressure space whereby foam and entrained gases are expelledfrom the centrifuge chamber by the pressure existing at the second impeller inlet, and third restricted conduit means communicating with the centrifuge chamber at an outer peripheral portion thereof for supplying gas-free liquid at substantially the full discharge pressure of the second impeller.

KENTON D. McMAHAN.

KENNETH A. DARROW.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,714,735 Skidmore May 28, 1929 1,985,799 Scheifele Dec, 25, 1934 2,370,778 Compton Mar. 6, 1945 2,485,525 Bedale Oct. 18, 1949 2,510,645 McMahan June 6, 1950 2,551,276 McMahan May 1, 1951 FOREIGN PATENTS Number Country Date 16,500 Denmark Dec. 21, 1911 630,932 Germany June 9, 1936 

